Victor Obot

Quasi permanent superconducting magnet of very high field

We report on persistent field in a quasipermanent magnet made of high temperature superconductor. The material has an average of 40% molar excess of Y, relative to Y1Ba2Cu3O7 and has been irradiated with high energy light ions at 200 MeV. The magnet, which traps 1.52 T at 77.3 K, traps nearly 4 T at 64.5 K. No evidence of giant flux jump or sample cracking was observed.

Permanent magnets of high‐Tc superconductors

Advances on permanent high‐Tc superconducting magnets are reported. Materials are tested in the form of small tiles. An accurate phenomenological model of the currents in a magnetized tile predicts that the maximum trapped field BT,max∝Jcf(d), where Jc is the critical current density, d is the diameter of the single‐grain tile, and f(d) is a function which increases monotonically with d. Results are reported of increasing Jc via chemical additives, and via bombardment by high‐energy light ions and fission fragments. Increases in d via chemical and temperature gradients are also reported. Methods, data, and most recent results are presented. Present values are d=2 cm, and Jc=85 kA/cm2, at 77 K. A six tile minimagnet, 1.2×1.2×1.5 cm3, fabricated from earlier tiles with d∼1 cm, Jc∼45 kA/cm2, retains 1.52 T at 77 K. It is calculated that the more recent values of Jc and d will result in fields of 3 T at 77 K. BT,max also increases rapidly with T−1, and approximately doubles at 60 K.

Materials, characterization, and applications for high Tc superconducting permanent magnets

Abstract A program for the development of materials and applications for high temperature superconducting permanent magnets has been pursued in this Institute. We have found that mapping of the magnetic field, plus use of a simple but accurate current model, permits easy measurement/calculation of Jc. This characterization method also shows grain structure simply and clearly, and is applicable to large size samples. The current model indicates that the maximum trapped magnetic field, BT,max, is approximately given by BT,max 8 Jcf(d), where f(d) is a monotonically increasing function of the diameter, d, of a single grain. Guided by this resultl our program aims to increase d, and Jc, in order to increase the field. We report on our progress. Currently non-irradiated materials with Jc 13,000 A/cm2, and irradiated materials with Jc up to 85,000 A/cm2 at 77 K are being applied, and developed further. Presently single grain tiles of ∼ 2 cm diameter in the a,b plane are produced. Tests these are in progress on the newest and best materials, and tests have been completed on earlier materials of smaller d, and lower Jc. Using these earlier materials, magnetic fields of 6,400 G have been retained on single grain irradiated tiles 1.2 × 1.2 × 0.4 cm3 and 2,200 G on single grain non-irradiated tiles of 2 cm diameter, at 77 K. A mini-magnet of dimensions approximately 1.2 × 1.2 × 1.2 cm, made up of 5 proton irradiated tiles, retained a field of 14,200 G at 77 K. A mini-magnet 1.2 × 1.2 × 0.6 cm retained a field of almost 2 T at 77 K. Creep has been studied for long term behavior in saturated samples, for unsaturated samples, and as a function of T, and irradiation. Tiles have been used to provide magnetic field for a motor with 19 Watt output.

Uranium fission fragment pinning centers in melt-textured YBCO

In the U/n process YBCO powders are doped with uranium, textured, and then irradiated with thermal neutrons. During texturing almost spherical deposits of (U 0.6 Pt 0.4 )YBa 2 O 6 (called Y-5), 300nm in diameter, are formed. Prior to irradiation Y-5 deposits increase J c about 10% per 0.1% M U , where M U is the percentage by weight of admixed uranium which contains an intimate mixture of both the 235 U and the 238 U isotopes. We find the number of Y-5 deposits « M U . Hence, inter-deposit spacing, S « M U -1/3 . Subsequent irradiation with thermal neutrons produces fission fragment defects that also act as pinning centers. The effective length for pinning, δ, of these pinning centers was estimated theoretically to be 2μm to 4μm. The effect of the chemical and irradiation pinning centers is multiplicative and increase J c , depending on applied field and temperature, by factors of 14 to 40. The value of F n at which J c peaks depends only on the product of F n and M U(235) , where F n is the number of neutrons per square centimeter. We find that optimum F n is inversely proportional to M U(235) . M U was varied from 0.0375% to 1.0%, with M U(235) held constant at 0.025%, thus varying S by a factor of 3. The enhancement of trapped field, R M , is 500% at 0.15%<M U <0.30% and drops to 400% for 0.0375% M U , indicating δ ∼ 2.7μm. Anisotropy of (3mm) 3 samples decreases monotonically from 3 to 2 with increasing F n . Residual radioactivity six months after irradiation is <200 nCi/g or <7400 Bq/g for samples with 0.025% M U(235) at optimum F n .

Radiation transport modeling and assessment to better predict radiation exposure, dose, and toxicological effects to human organs on long duration space flights

NASA is very interested in improving its ability to monitor and forecast the radiation levels that pose a health risk to space-walking astronauts as they construct the International Space Station and astronauts that will participate in long-term and deep-space missions. Human exploratory missions to the moon and Mars within the next quarter century, will expose crews to transient radiation from solar particle events which include high-energy galactic cosmic rays and high-energy protons. Because the radiation levels in space are high and solar activity is presently unpredictable, adequate shielding is needed to minimize the deleterious health effects of exposure to radiation. Today, numerous models have been developed and used to predict radiation exposure. Such a model is the Space Environment Information Systems (SPENVIS) modeling program, developed by the Belgian Institute for Space Aeronautics. SPENVIS, which has been assessed to be an excellent tool in characterizing the radiation environment for microelectronics and investigating orbital debris, is being evaluated for its usefulness with determining the dose and dose-equivalent for human exposure. Thus far. the calculations for dose-depth relations under varying shielding conditions have been in agreement with calculations done using HZETRN and PDOSE, which are well-known and widely used models for characterizing the environments for human exploratory missions. There is disagreement when assessing the impact of secondary radiation particles since SPENVIS does a crude estimation of the secondary radiation particles when calculating LET versus Flux. SPENVIS was used to model dose-depth relations for the blood-forming organs. Radiation sickness and cancer are life-threatening consequences resulting from radiation exposure. In space. exposure to radiation generally includes all of the critical organs. Biological and toxicological impacts have been included for discussion along with alternative risk mitigation methods--shielding and anti-carcinogens.

Self-assembling nano-diameter needlelike pinning centers in YBCO, utilizing a foreign element dopant

Although pinning centers created by irradiation presently produce the highest Jc, it is probable that ultimately these will be emulated by chemical pinning centers. The best pinning centers produced by irradiation nevertheless provide guidelines for desirable morphology of chemical pinning structures. The highest Jc produced earlier in textured HTS was obtained using isotropic high-energy ions produced by fission of 235U. This so-called U/n process produces pinning centers of diameter ≤ 4.5 nm, with an effective length of ~2.7 µm. Maximum Jc occurs for pinning center density of ~1010 cm−3. We use this as a model for desired chemical pinning centers. Our approach to introducing chemical pinning centers has been to produce precipitates within the HTS containing elements not native to the HTS, and to seek needlelike (columnar) deposits of small diameter. We report here on the formation of needlelike or columnar deposits in textured Y123 containing a dopant foreign to Y123. It serves as a demonstration that self-assembling nanometer diameter columns utilizing a dopant foreign to the HTS system are a feasible goal. These deposits, however, do not fully meet the ultimate requirements of pinning centers because the desired deposits should be smaller. The self-assembling columns formed contain titanium, are ~500 nm in diameter, and up to 10 µm long. The size and morphology of the deposits vary with the mass of admixed Ti dopant. Jc is decreased for small dopant mass. At larger dopant masses needlelike precipitates form, and Jc increases again. A small range of mass of admixed Ti exists in which Jc is enhanced by pinning. In the range of admixed Ti mass studied in these experiments there is a negligible effect on Tc. Magnetization studies of Jc are also reported.

Progress in Jc, Pinning, and Grain Size, for Trapped Field Magnets

Bulk Materials of high Jc and large single grain size are being developed for the goal of intense trapped field magnets (TFM). We report a new high field record, Btrap ≈ 7.0 Tesla, at 55 K. Data on temperature dependence of Bt, for 55 ≤T≤82 K, confirm a simple phenomenological equation for Jc(T) or Bt(T). We observe “training” phenomena in fabricated HTS TFM mini-magnets. We describe early results for pinning centers based on splayed columnar defects, which are created by the fission of seeded U in Y123. With these pinning centers, Jc ≈ 85,000 A/cm2 in large grains.

Nanometer Cerium: A Significant Improvement Over Platinum in Bulk YBCO Superconductor

Large grain YBCO superconductors, containing the same atomic percentage of Ce, and that of Pt, were produced via the melt-texturing process. Samples doped with nanometer cerium oxide exhibits an approximate 30% increase in trapped magnetic flux density, relative to submicrometer platinum doped YBCO. A 30% increase in trapped magnetic flux density represents ~ 80% increase in critical current density. Microstructure studies indicate that just like Pt, Ce refines Y211 particles to increase trapped magnetic flux density and critical current density. Cerium is much cheaper than platinum and is therefore, a desirable alternative to Pt in the large-scale production of bulk YBCO superconductors. Another interesting result of this experiment is that the trapped magnetic flux density of Ce-doped samples is unchanging over a large range of melting temperatures used for melt-texturing. In addition to refined Y211, Ce-rich deposits are also formed. The Ce-rich particles are insufficient in number to act as effective pinning centers.

Sub-Micron Non-Superconducting Deposits That Fail to Act as Pinning Centers in Textured YBCO Superconductor

A conundrum is observed where one type of small, non-superconducting particles fails to provide pinning, in textured YBCO superconductor, while other non-superconducting particles of comparable size and number density, pin very well. Pinning centers in high temperature superconductors (HTS) have been shown to greatly enhance critical current density (JC)- An experiment was performed in which large grain YBCO doped with uranium (U), but no platinum (Pt), was produced. Profuse deposits of a U-Y-Ba-O compound were formed during texturing. These U-rich deposits have an almost spherical morphology with an average diameter of ~300 nm and are uniformly distributed inside the YBCO. Conventional wisdom dictates that the U-Y-Ba-O sub-micron deposits should act as pinning centers and increase JC .However, in tests of JC via trapped magnetic flux density at 77 K, no increase was measured. This result is especially surprising because chemically similar and morphologically indistinguishable deposits of U-Pt-Y-Ba-O, found in (U + Pt)-doped YBCO, increased JC by a factor exceeding 2. The deposits compared had the same size and number density. Neither deposits significantly affected critical temperature (TC). One difference between these two compounds is that the non-pinning U-Y-Ba-O deposits have a single perovskite structure, while the U-Pt-Y-Ba-O pinning centers are a double perovskite. Data are presented on the remarkably similar morphology and strikingly different ability to trap magnetic flux density of the U-Y-Ba-O and U-Pt-Y-Ba-O deposits.